Conventionally, many rotating electrical machines have employed a single star connection. In this single star connection, taking for example forming eight U-phase coils, as shown in FIG. 4A, the coils are wound at U-phase coil arranging parts 1U to 8U, starting with a coil U1, winding coils U2 to U7 sequentially, and ending with a coil U8. Due to this arrangement, the coil U1 formed at the start of winding and the coil U8 formed at the end of winding are adjacent to each other. Here, the adjacent coils are wound in different directions. In U-phase coils, as shown in FIG. 4B, the potential difference becomes maximum at a point U and a neutral point. In other words, a maximum potential difference Vmax is caused between the coil U1 and the coil U8. This means that the single star connection brings the coils U1 and U8, wherein the maximum potential difference is caused, into adjacent to each other. Therefore, in rotating electrical machines with windings of the single star connection, insulation may not be ensured in between coils of the same phase when used at high potential.
To avoid this problem, a double star connection is employed instead of the single star connection in rotating electrical machines used at high potential (Japanese Unexamined Patent Application Publication No. 2002-199638). In the double star connection, taking for example forming eight U-phase coils in the same manner as above, a pair of coil groups, respectively consisting of coils U1 to U4 and coils U11 to U14, are arranged in parallel as shown in FIG. 5A. In the coil group of U1 to U4, as in the same manner as in the single star connection, the winding process starts with U1, moves to the coils U2, U3 one after another and ends with the coil U4. The coil group of U11 to U14 are wound in the same manner. Due to this winding manner, the coils U1 and U11 formed at the start of winding are adjacent to each other, and so are the coils U4 and U14 formed at the end of winding. As shown in FIG. 5B, a potential difference becomes maximum between the point U and the neutral point in the U-phase coils. This means that, the maximum potential difference Vmax is caused between the coils U1 and U4 and between the coils U11 and U14. In the double star connection, the coils U1 and U4, and the coils U11 and U14 are respectively located apart so as not to be adjacent to each other. Therefore, in the rotating electrical machines with the double star-connected windings, insulation in coils of the same phase can be secured even when used at high potential.